1. Field of the Invention
This invention relates to an accumulated-heat correction apparatus and an accumulated-heat correction method for the thermal head of a thermosensitive printer, a thermosublimation printer or the like.
2. Description of the Related Art
A prior-art accumulated-heat correction method of the type specified above will be described in due course.
FIGS. 6A-6C are conceptual diagrams in the case where accumulated heat is not corrected.
FIG. 6A shows a thermal-head conduction time. Usually, a thermal head comes to have a heat quantity corresponding to the conduction time. FIG. 6B shows a thermal-head temperature corresponding to the thermal-head conduction time in FIG. 6A. Usually, the density of a print is obtained in correspondence with the temperature of the thermal head. FIG. 6C exemplifies a print output generated by the thermal-head conduction time in FIG. 6A, and the central part thereof shows the states of the print densities. The conduction time of the (n−1)th line is tn−1, that of the nth line is tn, and that of the (n+1)th line is tn+1.
Referring to FIG. 6A, the conduction time tn−1 of the (n−1)th line and the conduction time tn of the nth line have an identical value tn−1=tn. However, heat is accumulated in the thermal head, so that when a current is conducted through the thermal head for equal time periods, a temperature rise corresponding to accumulated heat A occurs at the nth line as shown in FIG. 6B.
That is, notwithstanding that the (n−1)th line and the nth line are intended to be printed at an identical density, the density of the nth line becomes higher than that of the (n−1)th line in correspondence with the accumulated heat A on account of the heat accumulation of the thermal head.
Likewise, at the (n+1)th line, the conduction time is decreased to tn+1 with the intention of a print which does not develop any color, but an intended print density is not attained on account of residual heat corresponding to an accumulated heat B.
Next, a prior-art example in which accumulated heat is corrected is shown in FIGS. 7A-7C.
In the correction of the nth line in FIGS. 7A-7C, the conduction time of the nth line is decreased in correspondence with a correction A in FIG. 7A in order that the accumulated heat A of the nth line as shown in FIG. 6B may not incur the temperature rise.
The conduction time tn in FIG. 7A is obtained by subtracting the component of the correction A from the ordinary conduction time. Thus, as shown in FIG. 7B, the temperatures (or heat quantities) of the thermal head become substantially identical at the (n−1)th line and the nth line, and the intended density of the nth line is attained, so that the accumulated-heat correction is successfully made.
Likewise, in the correction of the (n+1)th line, the conduction time of the (n+1)th line is subtracted in correspondence with the accumulated heat B in FIG. 6B, whereby an accumulated heat quantity is corrected, and an intended print density is attained.
That is, the prior art is the accumulated-heat correction method which is schemed to attain the intended print density, in such a way that the accumulated heat quantity is predicted from data for which a current is to be conducted, and preceding data, whereupon the accumulated-heat correction is applied (refer to, for example, JP-A-2004-050563 and JP-A-2003-251844).
When the conduction time for the (n+1)th line is to be corrected by the prior-art accumulated-heat correction method as stated above, the accumulated heat B remaining at the (n+1)th line as indicated in FIG. 6B must be corrected. However, when the conduction time of the thermal head is to be subtracted as indicated at a correction B in FIG. 7A, it becomes less than zero. Since the conduction time does not become less than zero, a correction magnitude at the (n+1)th line becomes a correction B′.
That is, a correction magnitude is short by (the correction B− the correction B′) for the (n+1)th line, and the intended print density of the (n+1)th line cannot be attained.
Concretely, the prior art has had the problem that, although the density at which any color is not developed is intended for the (n+1)th line, the accumulated-heat correction cannot be fully made, resulting in a print density at which a color is somewhat developed.